This invention relates to new and useful improvements in clamps which are well suited not only for lifting heavy articles such as hull blocks in a shipyard or steel plates or sub-assemblies of a bridge or a building in a construction field, but also well suited for use as a pulling jig for pulling and holding articles in desired relative position for welding, bolting or riveting them together in such construction fields.
There has been known many types of clamps which are suited to be used in such fields mentioned in the above. One of the types of such clamps is a screw type which typically comprises a clamp body having a shackle for engagement with a pulling or lifting devices and a pair of leg portions integrally disposed on said body and forming a slot therebetween for receiving an article to be clamped, and at least a movable jaw comprising a screw rod movable to be advanced into and retreated from the slot for gripping or releasing an article within the slot.
In such type of clamps, it is desirable to provide means for preventing an article clamped from being slipped out of the clamp during operation, since it is often difficult to tighten the screw rod enough to hold the article or to confirm whether or not the screw rod has been tightened sufficiently to prevent the article from being slipped out when the clamp is lifted or pulled.
There has been proposed some solutions for this problem, for example U.S. Pat. No. 3,269,766; entitled "Screw Clamp with a Head having a Reverse-Acting and Renewable Gripping Surface"; granted to Mr. Edward Merril Gardner, and U.S. Pat. No. 3,947,011; entitled "Clamp Usable as Jig and Lifting Clamp"; granted to the same inventor of the present application.
In the clamps disclosed in these patents, there is provided a slidably and rotatably mounted gripping pad or a swivel jaw having a convex gripping surface which functions to provide a powerfully-wedged gripping action by the rotational movement of the gripping pad or swivel jaw when the clamp is lifted or pulled and the article gripped shows a tendency to be slipped out of the clamp.
However, there has been found a defect in these clamps that even though the article to be calmped is once firmly gripped, the screw rod often shows a tendency to be loosened due to a certain external force, for example, by vibration when such a force is applied to the clamp under a condition where no load is applied to the clamp or the article is gripped for a considerably long time.
Like phenomena are generally experienced in bolt and nut engagement. More specifically, bolt and nut engagement can maintain its locking or tightening condition when friction force between screw threads thereof is sufficient for preventing the nut from being loosened, but once a force for producing this friction is reduced to a certain extent, the nut tends to easily fall off the bolt by vibration.
In order to prevent the screw rod from being loosened, it may be thought that the lead angle of the screw to be formed in the rod should be rendered as smaller as possible, that is, a pitch should be maintained smaller so that the screw rod may resist an external force such as vibration, etc. However, in order to render the lead angle small, an angle of the screw thread (i.e. an angle between adjacent flanks of the screw threads) should consequently be maintained smaller, and this reduction of the angle of the screw thread is limited to a certain extent for bearing the load to be imposed upon the screw thread.
In fact, this loosening phenomenon has been observed, even in a clamp which is prepared in such a manner that the screw rod has a minimum angle of the screw thread in the light of the strength for bearing the load to be imposed upon the screw thread.
Therefore, it is a principal object of the present invention to provide a clamp which may ensure to prevent unintentional disengagement between the clamp and the article to be clamped irrespective of the clamp being applied a load or not.
Another object of the present invention is to provide a screw type clamp which is capable to prevent the screw rod from being loosened with the influence of unintentional external force such as vibration.
Still another object of the present invention is to provide a new and useful arrangement of a clamp which arrangement applicable not only to a lifting clamp and pulling jig, but also applicable to a connecting jig for fixing a free end of a safety rope appendant to a safety belt which is required by persons who work at high places for preventing them from falling down accidents.